很搞不懂，什么个结构啊

GWD-29-Q29

Most of Portugal’s 250,000 university students boycotted classes in a one-day strike to protest a law that requires them to contribute $330 a
            year toward the cost of higher education, previously paying $7 per year.

A.      year toward the cost of higher education, previously paying $7 per year

B.       year toward the cost of higher education, for which was previously paid $7 per year

C.      year, compared to the previously $7 per year, toward the cost of higher education

D.      year toward the cost of higher education, instead of the $7 per year required previously

E.       year as opposed to the $7 per year required previously for the cost of higher education

选D，C哪里错啊？